Frontal X-ray of the pelvis of a 22-year-old marathoner with painful hips and dependent on steroids. The scan shows vascular osteonecrosis of the femoral heads, more marked on the left (right side). Osteonecrosis is a condition is caused by poor blood supply to the affected area and the bone marrow. It most commonly affects the bones of the hips, knees and face. It can cause the bone and joint to collapse, and hip joints may need replacing or resurfacing.